1. Field of Invention
This invention is directed to the recovery of ammonia from waste streams. More specifically, this invention is directed to systems and methods for processing used chemical bath solutions to reclaim ammonia.
2. Description of Related Art
Semiconductor wafers, such as silicon wafers, are used for fabricating integrated circuits and solar cells, among other uses. Semiconductor wafer production is a multi-part process. First, in general, a semiconductor ingot is grown, often by the Czochralski method of pulling crystal, and shaped. The ingot is then sliced into wafers and prepared, through multiple steps, for polishing, in a process often referred to as the chemical wafer process. The wafer is then polished and inspected in the polished wafer process. Depending on end-use, the polished wafers are optionally subjected to a process by which an epitaxial layer is deposited on the wafer in the epitaxy process.
The use of highly purified ammonia in semiconductor wafer processing is well known. For example, U.S. Pat. No. 5,846,386 to Hoffman et al., U.S. Pat. No. 5,496,778 to Hoffman et al. and U.S. Pat. No. 5,242,468 to Clark et al. disclose the use of highly pure ammonia and liquid cleaners in the manufacture of semiconductor wafers and other electronic components.
During silicon wafer manufacturing, wafers are typically cleaned using solutions containing ammonia. Two common such solutions are SC-1 and SC-2 (Standard Clean) chemical bath solutions. SC-1 chemical bath solutions are highly basic and contain an oxidizing agent, usually hydrogen peroxide, H2O2, which must be treated prior to disposal. Specifically, SC-1 chemical baths contain high purity ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) and water (H2O). After several cleaning cycles, SC-1 chemical bath solutions will also include silicon, in the form of H2SiO3. However, SC-1 cleaning has its disadvantages. SC-1 solution is costly. In addition, including SC-1 cleaning in the process of manufacturing semiconductor chips requires the addition of steps for chemical and waste water treatment. Thus, SC-1 cleaning increases the complexity and the cost of semiconductor chip manufacture.
In conventional silicon wafer manufacturing processes, chemical bath solutions are treated before being discarded. Using SC-1 as an example, SC-1 chemical bath solutions are mixed with SC-2 chemical bath solutions, which are dilute solutions of hydrochloric acid (HCl) and hydrogen peroxide (H2O2). The combined solution is then neutralized with sulfuric acid (H2SO4) to produce ammonium sulfate ((NH4)2SO4) and water. Ammonium sulfate and water may be discharged safely into sewer systems, but environmental concerns remain and further processing may be necessary to avoid adverse environmental impacts in the future.
In addition, SC-1 chemical bath solutions are the source of significant expense in silicon wafer manufacturing processes. Very high-purity ammonium hydroxide for the SC-1 chemical baths and the sulfuric acid, used to neutralize the SC-1/SC-2 chemical bath combined solution, are expensive. Sewer fees are also costly.
Some reprocessing methods exist in the art for purifying certain liquids on an industrial scale. For example, U.S. Pat. No. 4,828,660 discloses a method for the reprocessing of ultrapure liquids, specifically sulfuric and peroxydisulfuric acids. The repurification disclosed is performed by withdrawing acid/oxidant solution from the processing stream, subjecting the acid/oxidant solution to, purification processes and reintroducing the purified solution into the process stream. However, there is no reprocessing or repurification method available, for recovering ammonia from ammonium-rich waste streams.
Thus, there is a need for semiconductor wafer processing systems and processes that eliminate the expense and waste problems associated with SC-1 cleaning.